P-T-T PATHS FROM THE SALMON RIVER SUTURE ZONE, WEST-CENTRAL IDAHO: CONTINENTAL GROWTH BY ISLAND ARC ACCRETION

The Salmon River Suture Zone (SRSZ) of west-central Idaho provides a unique glimpse into mid-lower crustal processes during continental growth by island arc accretion. The SRSZ separates island arc terranes of the Blue Mountains and the Mesozoic margin of North America. The rates, processes, and timing of terrane collision can be understood by studying metamorphism during crustal thickening. Prior work divided the SRSZ into two major structural blocks juxtaposed together by the Pollock Mountain fault: the overlying Pollock Mountain plate (PMp)- a polymetamorphic, polydeformational block dominated by amphibolite, and the underlying Rapid River (RRp) plate, a lower grade block dominated by phyllite and schist. Previous studies on the PMp suggest poorly understood pre-144 Ma metamorphism followed by metamorphism with a counter-clockwise P-T-t path [peak= 8-9 kbar ~600°C]. The latter path was inferred to include garnet growth during cooling and rapid uplift resulting from lithospheric delamination, at ca. 128 Ma. New P-T-t paths for the RRp constructed from Sm-Nd garnet geochronology, geothermobarometry, isochemical P-T phase diagram sections (pseudosections), and petrography suggest that slow cooling rates caused rim diffusion in garnet and retrograde temperature estimates producing a counter-clockwise P-T path. Garnet zoning suggests that similar diffusive re-equilibration may have occurred in the PMp. A new garnet age of 112.5±1.5 Ma from the RRp and the 128 Ma garnet age to the SW (Getty et al, 1993) suggest that contractional crustal thickening could have been prolonged and occurred between 130-100 Ma. The new 112.5 Ma age coincides with magmatism in the Hazard Creek and Little Goose Creek intrusive suites to the east (118-110 Ma). P-T paths indicate loading during garnet growth; therefore, magmatism just prior to peak metamorphism may have heated the rocks to peak T (~550-600° C) at intermediate pressure (~5 kbar) prior to garnet growth which continued during loading. This implies pre- to syn-metamorphic intrusion of the Hazard and Little Goose Creek suites during or prior to loading.